G. L. Ravera

Current drive at plasma densities required for thermonuclear reactors

Progress in thermonuclear fusion energy research based on deuterium plasmas magnetically confined in toroidal tokamak devices requires the development of efficient current drive methods. Previous experiments have shown that plasma current can be driven effectively by externally launched radio frequency power coupled to lower hybrid plasma waves. However, at the high plasma densities required for fusion power plants, the coupled radio frequency power does not penetrate into the plasma core, possibly because of strong wave interactions with the plasma edge. Here we show experiments performed on FTU (Frascati Tokamak Upgrade) based on theoretical predictions that nonlinear interactions diminish when the peripheral plasma electron temperature is high, allowing significant wave penetration at high density. The results show that the coupled radio frequency power can penetrate into high-density plasmas due to weaker plasma edge effects, thus extending the effective range of lower hybrid current drive towards the domain relevant for fusion reactors.

LOWER HYBRID CURRENT DRIVE FOR DEMO: PHYSICS ASSESSMENT AND TECHNOLOGY MATURITY

Abstract Recent experiments on lower hybrid (LH) penetration at reactor-relevant densities, together with the recent demonstration of the technological viability of the passive-active multijunction launcher on long pulses, have removed major concerns about the employment of LH waves on next-generation tokamaks, where LH could profitably drive far-off-axis plasma current, allowing current profile control and helping in sustaining burning performance. In this frame and with the aim of being prepared for the design phase of the next experimental reactors, preliminary investigations on the possibility of using LH on DEMO have been started under the supervision of the European Fusion Development Agreement. This paper reports the outcomes of these studies, addressing three main questions: Is LH useful for DEMO? If so, which setting of physics parameters makes it as effective as possible? Last, can available technology fulfill such demands? From the physics viewpoint, deposition sensitivity to launcher poloidal position, scrape-off layer parameters, and peak n‖ have been analyzed, indicating the equatorial injection of 5-GHz waves with n‖peak = 1.8 as the most favorable option. On the engineering side, specific research and development needs have been investigated on the basis of available information and sensible assumptions, showing that most of the components of the transmission line and, of highest priority, radio-frequency vacuum windows demand intense development.

Mode Filters for Oversized Rectangular Waveguides: A Modal Approach

A semi-analytical approach, relying on the mode-matching method and the resonator technique, is proposed. It is conceived to evaluate the performance of mode filters, based on corrugations partially filled with an absorbing material, in the case of oversized rectangular waveguides. The mathematical formulation allows an accurate and fast computation of the scattering parameters, through closed-form expression of the surface integrals and some matrix algebra. The theoretical model is implemented in a code that is benchmarked against a finite-element method to elucidate its advantages with respect to volumetric solvers. After giving an insight on the physical mechanisms ruling the performance of these devices, the modal method is used to run the particle swarm optimization algorithm for different types of devices, taking advantage of the reduced computation time of the developed tool.

On the use of corrugations in mode filters for oversized rectangular waveguides

The filtering of unwanted modes that can propagate in oversized rectangular waveguides is addressed, focusing on longitudinal corrugations partially filled with an absorber. Lengthwise slots, in the middle of top and bottom waveguide walls, can indeed extract the power of some modes from the main waveguide with negligible insertion losses for the working dominant mode. Once some power is coupled through the slot into the junction, it travels toward the absorbing material, where it is damped. This behavior is studied comparing the performance of a single continuous slot, where various modes can propagate, with the one achieved by several shorter single-mode apertures. The excitation and absorption of modes in the corrugations is estimated by means of analytical expressions under a small coupling approximation. Mono-modal corrugations achieve better performances with respect to the overmoded junction.

Curve concepts for oversized circular waveguides in view of a lower hybrid system for DEMO

Three candidates of waveguide bends for the transmission lines of Lower Hybrid systems in large-size tokamaks are assessed in terms of RF performance and integration issues. The three options are respectively characterized by profiled curvature, corrugated waveguide and elliptical cross-section; the unsatisfactory behavior of a standard, smooth wall, circular bend is also shown for comparison. The design of ideal curves achieving no mode conversion at their output is rather attended in literature because it has always been a critical issue for long-distance high-power transmissions at microwave frequencies. Here the problem is addressed taking advantage of modern numerical advancements and, unlike usual, focusing on small curvature radii. Analysis outcomes indicate the bend based on corrugated waveguide as the most suitable option from the viewpoint of both RF performance and integration issues. Details about adopted method and results are discussed.

Thin CVD-diamond RF Pill-Box vacuum windows for LHCD systems

The preliminary assessment of a Lower Hybrid Current Drive (LHCD) system for the DEMOnstration power plant (DEMO) is mainly focused on the R&D needs of the less conventional RF components of the Main Transmission Line (MTL) and of the launcher. 500 kW, CW klystrons will be used to deliver the RF power to independent Passive Active Multijunction (PAM) launcher modules at 5 GHz. This paper describes the criteria followed to investigate the optimum solution for the RF window used as vacuum barrier between the MTL and the launcher, an open issue in the LHCD system for ITER too. The best candidate, capable of withstanding a power level of, or above, 0.5 MW in CW operation and to satisfy the electrical and thermonuclear requirements, is a Pill-Box assembly, based on a thin single disk of CVD-diamond as dielectric, water cooled at the edge. A thickness of 3 mm, much shorter than half a wavelength of the TE°11 mode in the dielectric as in the conventional window (unfeasible and too expensive with CVD-diamond at t...

Physical Mechanisms and Design Principles in Mode Filters for Oversized Rectangular Waveguides

Mode filters, based on oversized rectangular waveguides with periodic symmetrical junctions, are addressed through analytical models under a small coupling approximation to explain the physical mechanisms of their behavior, and provide guidelines for their design. In the junctions, which are partially filled with a lossy dielectric, both propagating and evanescent modes play a role. Filter parameters are studied with respect to their impact on the excitation, propagation, and absorption of such modes in the junctions. Particular attention is paid to practical choices a designer has to face in real applications. The performance of single versus multiple lengthwise slots with the same total length is compared, and high-power matters such as voltage and thermal breakdown are discussed. A novel junction with two absorbing layers is proposed to help designers in maximizing absorption, while fulfilling thermal constraints.

MM-wave cyclotron auto-resonance maser for plasma heating

Heating and Current Drive systems are of outstanding relevance in fusion plasmas, magnetically confined in tokamak devices, as they provide the tools to reach, sustain and control burning conditions. Heating systems based on the electron cyclotron resonance (ECRH) have been extensively exploited on past and present machines DEMO, and the future reactor will require high frequencies. Therefore, high power (≥1MW) RF sources with output frequency in the 200 - 300 GHz range would be necessary. A promising source is the so called Cyclotron Auto-Resonance Maser (CARM). Preliminary results of the conceptual design of a CARM device for plasma heating, carried out at ENEA-Frascati will be presented together with the planned R&D development.

Corrigendum: Current drive at plasma densities required for thermonuclear reactors

Nature Communications 1: Article number: 55 (2010); Published: 10 August 2010; Updated:19 September 2013. In Fig. 3 of this Article, the colours of the blue and green curves were accidentally interchanged while the manuscript was being revised. In addition, the x axis labels on Fig. 4 should have read ‘Frequency (MHz)’.